1. Field of the Invention
This invention relates to a sunglass lens, more particularly to a color contrast enhancing sunglass lens comprising a lens body and a multi-layer coating.
2. Description of the Related Art
Besides conventional functions for attenuation of light and UV protection, along with the increasing demand for color contrast enhancement, the use of color contrast enhancing lenses has become increasingly popular in sunglasses. As used herein, the phrase “color contrast” refers to the difference in color that makes an object distinguishable. The image of an object can be enhanced by increasing the difference in light transmission between primary color bands (i.e., the blue band having a wavelength ranging from 430 nm˜470 nm, the green band having a wavelength ranging from 520 nm˜570 nm, and the red band having a wavelength ranging from 620 nm˜680 nm) and inter-primary color bands (i.e., the cyan band having a wavelength ranging from 470 nm˜520 nm, the yellow band having a wavelength ranging from 570 nm˜590 nm, and the orange band having a wavelength ranging from 590 nm˜620 nm). Therefore, a sunglass lens capable of enhancing the contrast of primary color will have a spectral profile with at least one high transmission band in the wavelength range of primary color bands, and at least one low transmission band in the wavelength range of inter-primary color bands.
To maximize the color contrast effect of a sunglass lens, some techniques for creating relatively steep sloped spectral profile have been exploited. These techniques include applications of a rare-earth doped glass material and a multi-layer coating to the sunglass lens.
U.S. Pat. No. 6,604,824 issued to Larson discloses a rare-earth doped polarized lens system. The spectral transmittance profile of a neodymium added lens wafer (1.0 mm thick) used in Larson' s lens system is shown in FIG. 1. During preparation of the lens wafer, after neodymium oxide (Nd2O3) is melted in a glass matrix, the Nd3+ ions have strong absorption in the yellow band at about 584 nm. Thus, the Larson lens system enhances the contrast of red and green to yellow. However, the Larson lens system shows no low transmission band in the cyan band centered at about 500 nm for enhancing the contrast of blue and green to cyan. Therefore, the rare-earth doped lenses can enhance the contrast of only part of but not all of the three primary colors.
In contrast to the application of a rare-earth doped glass material, use of a multi-layer coating provides a more flexible method in spectral profile design. U.S. Pat. No. 5,646,781 issued to Johnson Jr. discloses an optical filter for forming an enhanced image made by stacking a multi-layer coating on a substantially transparent substrate. The spectral profile of the filter of Johnson Jr. is designed to block passbands substantially centered at 490 nm and 590 nm. The spectral transmittance profile of a filter of Johnson Jr. that is used for the human eye is shown in FIG. 2. The filter shows strong primary color contrast. However, some serious problems prohibit the filter of Johnson Jr. from being a useful sunglass lens. First, the filter blocks the transmittance of light at 490 nm and 590 nm, and thus, it fails to meet the ANSI specification Z80.3-2009 section 4.6.3.3 for traffic signal recognition, which requires the spectral transmittance of a tinted lens to be not less than 20% of the lens luminous transmittance between 475 nm and 650 nm. Moreover, when used under bright sunlight condition, the filter made from the transparent substrate and the multi-layer coating produces unpleasant reflection to incident light from behind the wearer or the wearer' s face. The wearer will see the reflected image from the rear surface of the filter. Furthermore, the filter allows relatively high transmission of orange light at 610 nm and 620 nm, and thus is not good for enhancing the contrast of red from orange. Meanwhile, the filter suppresses the transmission of yellow green light at 560 nm, and thus, the color of green plants looks more bluish, a sense of reduced saturation of green.
Therefore, there is an unfulfilled need for a color contrast enhancing sunglass lens that simultaneously provides color contrast enhancement to all three primary colors without undesired reflection, complete UV protection, image sharpness improvement by blocking short-wavelength visible light between 380 nm and 410 nm, and meets the requirements of traffic signal recognition as defined by ANSI Z80.3-2009 section 4.6.3.